We have used immersed boundary simulations to explore the effect of scale on the efficiency of locomotion. For Reynolds numbers below 10, swimming efficiency drops drastically for jellyfish using jet propulsion and paddling mechanisms of locomotion. This range corresponds to the lower Reynolds number limit of pulsing jellyfish observed in nature.
APS DFD Movie on Upside Down Jellyfish Fluid Dynamics
Experiments with Jellyfish
Understanding how morphology of medusae affect the hydrodynamics of feeding, nutrient and gas exchange is important for distinguishing foraging mechanisms. We are quantifying the differences between the currents generated by oblate jellyfish that rest on the ocean floor and by free-swimming medusa. The phase-averaged flow generated by bell pulsations is similar to a vertical jet with induced flow velocities on the order of 1-10 mm/s. The bell margin is pushed against the substrate during full contraction in a manner not observed in free-swimming medusae kinematics, This introduces a strong, near-horizontal entrainment of the fluid toward the oral arms unlike the mostly upstream entrainment observed in free-swimming medusae. The resulting net fluid motion is not dominated by flow-reversal regions, suggesting that Cassiopea brings in new fluid with each bell pulse.
Numerical simulations of simplified models of jellyfish are used to explore the particle exchange mechanism associated with bell pulsations. Numerical models allow the ability to isolate fundamental effects of different parameters as well as to extend the examination beyond experimental feasibility or even biological possibilities. In this way computational studies present an opportunity to explore fundamental physical limits on biological mechanisms. Please see the movies below for a sample of our work on the flows generated by upside down jellyfish!